It is advantageous to increase the polymerization rate of a photopolymerizable composition, namely to increase the photosensitivity of the photopolymerizable composition, for shortening the time of exposure to an actinic radiation. Various processes have been proposed for this purpose from old times.
For example, a photoinitiator or photosensitizer is added to a photopolymerizable composition comprising an ethylenically unsaturated compound in order to improve the photosensitivity thereof. The photoinitiators or photosensitizers thus used include polynuclear quinones such as 2-ethylanthraquinone and 2-tert-butylanthraquinone, aromatic ketones such as benzophenone and 4,4'-bis (dimethylamino)benzophenone, and benzoin derivatives such as benzoin methyl ether.
However, the photosensitivity of the ordinary photopolymerizable composition containing such a photoinitiator or photosensitizer is not always sufficient.
Japanese Patent Publication No. 27605/1978 discloses an acridine or phenazine compound which may contain a condensed benzene ring as a photoinitiator having a high sensitivity particularly in the presence of oxygen and an excellent storability. Japanese Patent Laid-Open No. 226002/1984 discloses a combination of 9-phenylacridine with a thiol compound as a photoinitiator having a high sensitivity. Japanese Patent Laid-Open No. 164739/1985 discloses a substituted 9-benzoylacridine as a photoinitiator having a high sensitivity. However, no photopolymerizable composition having a satisfactory photosensitivity could be obtained even by using such a compound used heretofore.
Therefore, an object of the present invention it to provide a new photopolymerizable composition having an improved photosensitivity and a photopolymerizable element prepared therefrom by overcoming the defects of the prior art.